Re: [linux-lvm] ANNOUNCE: an experimental implementation of snapshot merging

[Mikulas Patocka <mpatocka@redhat.com>]

On Wed, 4 Jun 2008, Steeve McCauley wrote:

> Is this change meant to add cluster support for merging?  I had
> been under the assumption that merging was already a feature of
> snapshots.

My implementation has nothing to do with clustering.

I think there were some userspace programs for off-line merging of 
inactive snapshots. My implementation works in kernel and can be used 
while the snapshot is mounted.

> What is the difference between the current implementation using
> lvremove (which I assumed was where the COW blocks were merged back
> into the original volume) and lvconvert as below?  Why was this
> functionality not kept in lvremove?

lvremove drops the snapshot and forgets its content. lvconvert -M copies 
the snapshot to the origin (I'm now working on to let it automatically 
drop the snapshot when the merging finishes).

Mikulas

> Mikulas Patocka wrote:
>> Hi
>> 
>> Here I release the first experimental implementation of snapshot merging. 
>> Merging allows you to copy data in snapshot back to the origin device. Once 
>> merging starts, it runs on background. When the merging finishes (you see 
>> "0%" with "lvs" command), you should remove the merging snapshot with 
>> lvremove command.
>> 
>> The userspace will still be changed, the kernel is already finished (there 
>> are no known bugs in the kernel, I'm curious about the unknown ones :)
>> 
>> Merging is initiated with "lvconvert -M vg/lv_snapshot" command. When you 
>> initiate merging, make sure that the origin device is not mounted. Snapshot 
>> can be mounted while it's merging is initiated.
>> 
>> During merging, aby reads and writes to the origin device are identical to 
>> accesses to the merging snapshots.
>> 
>> There may be multiple snapshots while one of them is being merged --- 
>> exceptions in other snapshots are being allocated and there snapshots are 
>> kept stable.
>> 
>> Don't try to concurrently merge more than one snapshot (the kernel refuses 
>> to do it, the userspace tools don't know about it --- the userspace will be 
>> rewritten anyway).
>> 
>> The patches are at:
>> http://people.redhat.com/mpatocka/patches/
>> The patches are for 2.6.26-rc4
>> 
>> (besides merging, there are some other bugfixes, I made for the kernel)
>> 
>> Mikulas
>> 
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>
> -- 
> Steeve McCauley
> Director of Research and Development
> Rackable Systems
> 407 McGill Street, Suite 1002
> Montreal, Quebec, H2Y 2G3 (Canada)
> Tel: 514-288-3343
> http://www.rackable.com
> :wq
>
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